1. Field of the Invention
The invention disclosed herein relates to nuclear magnetic resonance measurements of earth formations penetrated by a borehole.
2. Description of the Related Art
Accurate and precise measurements of properties of materials are necessary for the efficient exploration and production of hydrocarbons. One measurement technique makes use of the unique nuclear magnetic resonance (NMR) characteristics that the materials possess.
NMR measurements are generally performed in a borehole penetrating earth formations that may contain reservoirs of the hydrocarbons. This technique is referred to as NMR well logging.
In NMR well logging, an NMR logging instrument (i.e., tool) is conveyed through the borehole by a wireline or drill string for logging-while-drilling applications (LWD). NMR measurements are performed at various depths to produce a log. Thus, a petro-physicist or log analyst reviewing the log can form an image of and quantify the various materials and rock properties downhole.
NMR instruments measure the abundance of a nucleus under investigation (such as a proton) in the NMR instrument's sensitive volume and the evolution of nuclear spins, such as relaxation and fluid molecule self-diffusion processes. In well logging applications, quantification of multiphase fluids (i.e., different fluids separated from each other) in a porous rock formation often depends on the contrast of one or more MMR properties of individual fluid phases saturating the porous rocks. These properties include, but are not limited to, proton density, longitudinal relaxation time T1, transverse relaxation time T2, and diffusivity. Thus, if the fluid property contrasts marginally or overlaps between the different fluid phases, discerning the different fluids becomes quite difficult if not impossible. For example, light hydrocarbons and water in large pores have comparable relaxation times and diffusivity and are therefore difficult to quantify and to discern from one another.
Therefore, what are needed are techniques used in NMR logging that can discern different materials from each other when the materials have comparable NMR properties.